The present invention relates generally to low refrigerant level detection and more particularly to such detection in automotive air conditioning systems.
Automotive air conditioning systems employ a mixture of refrigerant and oil, hereinafter referred to as refrigerant charge A compressor receives necessary lubrication from the oil in the refrigerant charge and circulates the refrigerant charge through the system's condenser, expansion valve and evaporator in a refrigeration cycle. Should the refrigerant charge drop in volume due to a system leak, improper service or otherwise, the compressor may not receive sufficient lubrication for continued operation and will eventually become damaged if allowed to run. It is recognized, therefore, that low refrigerant charge detection is desirable as part of an air conditioning system which is responsive thereto to shut the system off prior to the compressor becoming damaged. Typically, the compressor is driven by the automobile's engine and coupled thereto via an electromagnetic cycling clutch which engages the compressor to the engine. A low refrigerant charge typically causes disengagement of the cycling clutch to prevent compressor damage.
Some systems employ a variable displacement compressor which can be destroked to account for a certain amount of refrigerant charge loss which might otherwise require compressor shut down; however, the amount of destroke is finite and refrigerant charge loss beyond a certain amount will still require compressor shut down to avoid damage. The primary purpose of displacement variability is to control system cooling capacity and in practice has little effect as protection from a low refrigerant charge. Variable displacement compressors, therefore, are also used in conjunction with a cycling lutch as part of low charge protection. Variable displacement compressors are typically more costly than fixed displacement compressors for equivalent cooling capacities and also require multiple point sensing of system pressures and temperatures for adequate system control.
Other systems using fixed displacement compressors employ the cycling clutch to control cooling capacity by running the compressor intermittently in response to detected temperature and/or pressure parameters of the system. At least one such system has a fixed-size expansion valve orifice and, when operating with a proper refrigerant charge, is characterized by dependent pressure and temperature on the low-side (suction side) of the compressor. This type of system utilizes a low-side pressure transducer to cause the engagement and disengagement of the cycling clutch to control cooling capacity of the system. If the refrigerant charge becomes too low, low pressure and high temperature results on the low-side of the compressor. Low pressure in this area in a properly charged system would indicate low temperature, which indicates the need for less cooling capacity. The low pressure therefore causes more frequent cycling of the clutch and less on time of the compressor. Increased cycling at a low refrigerant charge and corresponding higher temperatures enhances compressor durability. However, cycling the clutch to control cooling capacity in a properly charged system tends to place uneven loads upon the engine which reduce fuel efficiency; further, frequent cycling may become annoying to some drivers. It is also known that systems with fixed expansion valves vary in cooling efficiency since the evaporation rate of the refrigerant charge therethrough is optimal within only a relatively small window of operating conditions.
Still a further fixed displacement compressor system utilizes a variable expansion valve to optimize refrigerant charge expansion therethrough for increased cooling efficiency throughout a larger window of operating conditions, to reduce compressor cycling for controlling cooling capacity, and to smooth compressor loads upon the engine. However, in systems utilizing a variable expansion valve, even with a proper refrigerant charge, low-side pressure and temperature are not always dependent since pressure may change rapidly with changing expansion valve orifice size; therefore, a pressure transducer will not appropriately indicate low side temperature as it would in a system having a fixed expansion valve orifice. Such a system typically utilizes a low-side temperature transducer to cause the engagement and disengagement of the cycling clutch to further control cooling capacity of the system. If the refrigerant charge becomes too low, the resulting elevated temperature on the low-side of the compressor would, without further control, cause reduced cycling and more compressor on time. Reduced cycling at low refrigerant charge will eventually lead to compressor damage.